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Old 10-09-2019, 03:34 PM   #11
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Originally Posted by Bouli306 View Post
Your statement is incorrect. The JFS can and will be used in flight.

Normal procedures for an airstart below 20000ft is to start the JFS. The JFS will take care of motoring the engine so the pilot will not have to worry about maintaining airspeed to keep the RPM of the windmilling engine within limits for an spooldown airstart, and can start thinking about preserving airspeed (and therefore altitude). When the JFS is started the ADG is run so also the Hyd B pump. Therefore it may be possible to lower the landing gear by simply lowering the gear handle.
It can be used in flight to facilitate an air-start. That's different. The JFS also does NOT provide enough torque to drive the ADG at any sort of speed where the A or B-pumps deliver any pressure, regardless of whether you're in the air or on the ground.

If you have to use the JFS to windmill the engine, you're already on EPU due to the CSG going offline, and the EPU is what's driving the A-system. At a lower pressure as well, I might add.


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Originally Posted by Bouli306 View Post
The EPU will indeed give elec power and Hydraulic A power. (by the way you forgot to mention the LEF's because they are secondary flightcontrols and run by A (and B) system as well.
Correct, I should have said both primary and secondary flight-controls. The secondary meaning the LEF's and the speedbrakes.

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Originally Posted by Bouli306 View Post
EPU has nothing to do with lowering the landing gear. When hydr B system is gone the pilot will lower the landing gear handle and pull the Alternate gear handle to lower the gear with nitrogen pressure. When the JFS is running he will have wheleebrakes on landing. When the JFS is not running he still has around 75 seconds of braking action to his availability by hydr accumulators. When alternate landing gear is used NWS is not available.
I never claimed that the EPU had anything to do with lowering the gear. The nitrogen pressure in the JFS accumulators will blow open the MLG doors and unlock it, the airflow will handle the rest to lock them in place. The NLG will need to be extended fully due to the nitrogen, as it opens against the airflow.

Again, the JFS WILL NOT drive any of the hydraulic systems during operation. Please state sources for this, and I'll agree with you.

The braking ALSO comes from the accumulators, but from the hydraulic section of it. It will, as you say, provide braking-power for a short while.
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Old 10-09-2019, 05:59 PM   #12
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Originally Posted by ShadowXP View Post
It can be used in flight to facilitate an air-start. That's different. The JFS also does NOT provide enough torque to drive the ADG at any sort of speed where the A or B-pumps deliver any pressure, regardless of whether you're in the air or on the ground.
Well...let me start by saying that in a earlier post you said "JFS is not run in flight, and wouldn't help you even if it could". And now you are saying that's different. The JFS DOES provide enough torque to drive the ADG. Even on ground the JFS will start pressurizing/filling the JFS/Brake accumulators 2 seconds before the runlight goes on till about 40-60 seconds after the JFS runlight where it completely replenished those accumulators on around 3250PSI pressure. That pressure is coming from the B pump. It is also stated in the Flight Manual.

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Originally Posted by ShadowXP View Post
If you have to use the JFS to windmill the engine, you're already on EPU due to the CSG going offline, and the EPU is what's driving the A-system. At a lower pressure as well, I might add.
You are not using the JFS to windmill the engine. You are using the JFS to mechanically mill the engine via the ADG, PTO shaft, Towershaft. True, the EPU will kick in and indeed the EPU will drive the A pump. I do not agree totally on your statement that the EPU pump provides lower pressure. The normal pumps will provide between 2850 and 3250PSI pressure, the EPU pump will give 3000PSI pressure.

The big difference between the regular A pump and the EPU pump is the flow it can produce. The EPU pump can only make around half of the flow of the regular pump (~42GPM against ~23GPM)

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Originally Posted by ShadowXP View Post
I never claimed that the EPU had anything to do with lowering the gear. The nitrogen pressure in the JFS accumulators will blow open the MLG doors and unlock it, the airflow will handle the rest to lock them in place. The NLG will need to be extended fully due to the nitrogen, as it opens against the airflow.
Okay, point taken about the EPU. But the Alternate landing gear extension pressure is NOT supplied by nitrogen coming from the JFS/BRAKE accumulators. The nitrogen in these two bottles is purely to put the accumulators under a certain precharge. The Alternate landing gear pressure comes from the nitrogen bottle just aft of the nose landing gear bay. That bottle is also used for the hook extension. And indeed the MLG is freefall by old man gravity and the airflow, airflow is secondary because also on the ground on jacks the gear will fall down and locked....and as far as I know, no airflow there apart from the mechanic letting one go

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Again, the JFS WILL NOT drive any of the hydraulic systems during operation. Please state sources for this, and I'll agree with you.
Yes it will and that is why I asked the original poster if the JFS was running. Although the JFS can't supply enough pressure to fly the jet (FLCS operation) it is known that gear came down only on JFS power. As stated above the Flight manual states that the JFS will replenish it's own accumulators to around 3250PSI.

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The braking ALSO comes from the accumulators, but from the hydraulic section of it. It will, as you say, provide braking-power for a short while.
JFS start power (hydraulic) and Braking (hydraulic) is supplied by JFS/Brake accumulators (what's in a name). But if the JFS runs on landing you can brake all you want. 75 seconds of braking during a regular landing is quite long actually, especially when you perform a good aerobrake.
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Old 10-09-2019, 06:27 PM   #13
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I agree with Bouli
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Old 10-09-2019, 07:51 PM   #14
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Originally Posted by Bouli306 View Post
Well...let me start by saying that in a earlier post you said "JFS is not run in flight, and wouldn't help you even if it could". And now you are saying that's different. The JFS DOES provide enough torque to drive the ADG. Even on ground the JFS will start pressurizing/filling the JFS/Brake accumulators 2 seconds before the runlight goes on till about 40-60 seconds after the JFS runlight where it completely replenished those accumulators on around 3250PSI pressure. That pressure is coming from the B pump. It is also stated in the Flight Manual.
First of all, the outlined section tells me that you've never worked a day on a Viper flightline. No Viper either replenishes the reservoirs that fast, nor do they replenish the reservoirs from the JFS.

How do I know? Because I've had to re-pressurize the damn reservoirs more than once through the manual T-handle in the left wheel well on the AM/BM's I served on, and that mirrors what other present-day and former Viper C-groundcrew say. Mvsgas on these boards is one of them. A mis-start will empty the bottles if the engine fails to light off, depending on whether or not you used START 1 or START 2. A START 1 that fails MIGHT have enough pressure for one more attempt, a START 2 will leave your arms and torso aching after pumping.

Also, please state the pages in the flightmanual that states that the JFS can and will pressurize the B-system while it's running. I'll wait.
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Old 10-09-2019, 08:33 PM   #15
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Originally Posted by ShadowXP View Post
First of all, the outlined section tells me that you've never worked a day on a Viper flightline. No Viper either replenishes the reservoirs that fast, nor do they replenish the reservoirs from the JFS.

How do I know? Because I've had to re-pressurize the damn reservoirs more than once through the manual T-handle in the left wheel well on the AM/BM's I served on, and that mirrors what other present-day and former Viper C-groundcrew say. Mvsgas on these boards is one of them. A mis-start will empty the bottles if the engine fails to light off, depending on whether or not you used START 1 or START 2. A START 1 that fails MIGHT have enough pressure for one more attempt, a START 2 will leave your arms and torso aching after pumping.

Also, please state the pages in the flightmanual that states that the JFS can and will pressurize the B-system while it's running. I'll wait.


Never worked a day on a Viper Flightline. Give me a brake buddy I worked the flightline and worked in corrective maintenance for about 9 years in the RNLAF. Therafter i became a Flight sim instructor on the F-16 which I still am.

Reservoirs is simply the wrong name, we are talking accumulators. But you know this... And sure the T-handle is a lot of work (around 220 stroke 'so to speak') but look at that little pump in the Lefthand main wheel well compared to a 450HP jet engine!!! By the way we have a pneumatic tool which we use to replenish the JFS/Brake accumulators.

You seem to be talking about a JFS start fail on ground..than you are committed to recharge the accumulators.

When the engine mills (JFS assisted or not) above 12% the JFS/BRAKEaccumulators are replenished in about a minute (faster than your handjob in that main wheel well :-)). It does this via the B pump on the ADG. It simply has no other means to do so.


Page 3-79 chapter airstart in a flight manual:
Allow 1 minute of engine rotation (either windmilling or JFS assisted) at 12 percent rpm or above to insure that the brake/JFS accumulators are fully recharged.


Recharging begins 3-4 seconds before the JFS RUN light illuminates or 30 seconds after selecting a start position.

And yes....also on the ground it will replenish that fast. Just stick your head in the hatch where the JFS/BRAKE gauges are during startup....al kind of magic going on there (gauges going from pre-charge to 3250ish in about a minute)

See yah

Last edited by Bouli306; 10-09-2019 at 09:02 PM.
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Old 10-09-2019, 09:06 PM   #16
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Yeppp,,,What he said..
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Old 10-10-2019, 05:00 AM   #17
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Never worked a day on a Viper Flightline. Give me a brake buddy I worked the flightline and worked in corrective maintenance for about 9 years in the RNLAF. Therafter i became a Flight sim instructor on the F-16 which I still am.
I'll consider myself sufficiently humbled, then. My big mouth has a tendency to get me into trouble about as fast as my stubbornness.

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Originally Posted by Bouli306 View Post
Reservoirs is simply the wrong name, we are talking accumulators. But you know this... And sure the T-handle is a lot of work (around 220 stroke 'so to speak') but look at that little pump in the Lefthand main wheel well compared to a 450HP jet engine!!! By the way we have a pneumatic tool which we use to replenish the JFS/Brake accumulators.
Weird, because all the info I've got, was taught and have had others tell me is that the JFS has a nominal power output of about 220bhp, not 450. Recharging the accumulators pneumatically was something we never did, mostly because it's far easier to just recharge them through muscle-power than it is to fetch tools and gear needed to do it. Plus, as you say, it's a good workout. Sucks to have the acc's at 850-900psi after a mis-start.

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Originally Posted by Bouli306 View Post
You seem to be talking about a JFS start fail on ground..than you are committed to recharge the accumulators.

When the engine mills (JFS assisted or not) above 12% the JFS/BRAKEaccumulators are replenished in about a minute (faster than your handjob in that main wheel well :-)). It does this via the B pump on the ADG. It simply has no other means to do so.


Page 3-79 chapter airstart in a flight manual:
Allow 1 minute of engine rotation (either windmilling or JFS assisted) at 12 percent rpm or above to insure that the brake/JFS accumulators are fully recharged.


Recharging begins 3-4 seconds before the JFS RUN light illuminates or 30 seconds after selecting a start position.


Yeah, realized that I'm hung up on on groundstarts. My bad.

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Originally Posted by Bouli306 View Post
Quote:
Originally Posted by Bouli306 View Post
And yes....also on the ground it will replenish that fast. Just stick your head in the hatch where the JFS/BRAKE gauges are during startup....al kind of magic going on there (gauges going from pre-charge to 3250ish in about a minute)

See yah
Weird, because again, we were taught to never use the JFS to get the accumulator-pressure back up to normal. If that was because of wear and tear on the JFS itself or not I don't know, but that's at least what we were taught. And also what other groundcrew, mostly USAF, has told me.

Either way, sorry for being a bit of an ass about this. I'm far too stubborn for my own good at times.
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